Matthew 5:23,24 (Luther)

Matthew 5:23. So if you are offering your gift at the altar, and there remember that your brother has something against you,
24. leave your gift there before the altar and go; first be reconciled to your brother, and then come and offer your gift.

He delivers a long sermon on this commandment, which seems to be an easy text. But this vice is very widespread and common, especially among people who are high and mighty and wise, like the people at the courts of kings, lords, and princes. Whoever amounts to something and is in a position to do something on earth, becomes involved in this. Yet this dare not be called by name, since it puts on a very good front and gets itself all polished and shined with a show of sanctity. In this way many people deceive themselves and others. They do not see that in their heart they hate their neighbor or bear a secret grudge against him. Yet they want to give the appearance of being pious, of serving God; and as He says here, they go to the altar and offer a sacrifice, supposing that everything is all right with them. This is how it works. There is a decorative and beautiful screen called “zeal for righteousness,” a virtue which loves justice, which hates wickedness and cannot bear it.30 Thus the sword and the government were ordained to administer righteousness and to punish wickedness, just as father and mother and master and mistress have to get angry and punish. Now along comes a pious scoundrel. He puts on his little mantle and says that he is doing it out of love for righteousness and that he has a just and appropriate cause for doing so. At the present time certain princes and others are crammed full of poison, hatred, and envy against our supporters. So saturated are they with this that they have no conscience about it, and the whole issue is a matter of “indulgences” and “shrines.” They protect themselves beautifully with the screen of heresy-hunting, making it a great virtue, a holy zeal, and a love for truth. At bottom there is nothing but a shameful hatred and a poisonous spite, which has no other way of manifesting and expressing itself.
With the exception of our dear lord the emperor personally, who has not received any better information about us, none of our opponents has or knows any reason to hate us and oppose us except sheer envy and malice. This I know and dare to say: They are not acting this way on account of any crimes that we have committed or because we are such rascals or scoundrels or because we have done them any harm. They know, too, and have had to admit that our doctrine is completely true. So poisonous are they, nevertheless, that they would tolerate a world full of nothing but desperate scoundrels rather than us and our supporters.
Many people who are otherwise fine, respectable, learned, and upstanding become filled with secret anger, envy, and hate, and are embittered by it. Still they never become aware of it, and their conscience is satisfied that what they are doing is in pursuit of their office or in obedience to righteousness. Their screen is so lovely and delusive that no one dares to speak of them as anything but pious and upstanding people. The ultimate result is a sin against the Holy Spirit and hardened hearts, which become confirmed and obdurate in this poisonous vice. There are two aspects to this wickedness. In the first place, the heart is full of anger, hate, and envy. But in the second place, it refuses to admit that this is sin and malice, but wants it to be called virtue; this amounts to slapping God across the mouth and calling Him a liar in His words.
You see, that is why Christ warns everyone so diligently to be on the lookout here and not to be fooled by this hypocrisy and pretense. It is incredible that such a simple bit of instruction can be so far-reaching and strike such great people. By the words “if you are offering your gift at the altar” He makes it clear that He is talking about people who serve God and claim to be His true children, who have a reputation as paragons of virtue. What is wrong with them, then? Nothing at all, except that their heart is crammed full of hate and envy! My friend, what is the use of continual fasting and praying, of giving away everything you have for God’s sake (1 Cor. 13:3), of whipping yourself to death, and of doing twice as many good works as all the Carthusians put together if meanwhile you ignore the commandments which God wants you to obey? Does it not bother your conscience to slander and defame other people and at the same time to offer a great sacrifice? That is the same as bringing on war, murder, and bloodshed—and then paying a thousand guldens to have Masses said for the souls of those who were killed; or stealing a large amount of money—and then giving alms for God’s sake. In this way they deceive God as well as themselves with their pretty pretense, and they imagine that now He has to consider them real living saints.
Therefore He says now: “If you intend to serve God and to offer a sacrifice, but are guilty of harming someone or of being angry with your neighbor, you should know immediately that God wants no part of this sacrifice. Lay it right down, drop everything, and go straight to your brother to be reconciled.” With the term “sacrifice” He is referring to every possible work done in the service or to the praise of God, since at that time offering a sacrifice was the best possible work. He rejects it completely, demanding that you leave it unless your heart tells you beforehand that you are reconciled with your neighbor and unless you are unaware of any anger against him. “If this is so,” He says, “come and offer your sacrifice.” He appends this to avoid the impression that He wants to reject or despise such a sacrifice, which was not an evil deed, but one that God had ordered and commanded; what was evil and what ruined it was their disregard and contempt for His other and higher commandments. That amounts to abusing sacrifices to harm your neighbor.
Another and more serious abuse of sacrifice is trying to be saved through it, using it as atonement for sin and as a foundation for our confidence and trust before God; we have discussed this elsewhere. In itself it is a good work. It is not right to despise or neglect any of the other works of public worship, like praying and fasting, where they are intended and used properly (that is, not in order to merit heaven) and where the heart is right with its neighbor. In this way the condition of both faith and love is pure and right. But if you pray and fast and at the same time you gossip about your neighbor or defame and slander people, your mouth may be speaking holy words and eating nothing, but meanwhile it is polluting and defiling itself with your neighbor contrary to God’s commandment.
In the prophet Isaiah, therefore (Is. 58:3–7), He denounces and forbids the fasting with which they punished their bodies and made a pretense of great devotion; and He says: “Behold, in the day of your fast you seek your own pleasure and oppress all your debtors. You fast only to quarrel and to fight and to hit with wicked fist. Fasting like yours this day will not make your voice to be heard on high.” He goes on to teach them about the meaning of proper fasting: “This is a fast that I have chosen. Release those who are in unjust bondage to you, and let those whom you are oppressing go free. Share your bread with the hungry; when you see the naked, cover him.” Here you see that all He cares about is love to the neighbor.
Luther, M. (1999, c1956). Vol. 21: Luther's works, vol. 21 : The Sermon on the Mount and the Magnificat (J. J. Pelikan, H. C. Oswald & H. T. Lehmann, Ed.). Luther's Works (21:79). Saint Louis: Concordia Publishing House.